A technique for detecting an uneven pattern on a surface of an object is used for detecting fingerprints.
Conventionally, following three methods are mainly known as a method for detecting fingerprints. A first method is a capacitance method in which a capacitance is formed between a predetermined electrode in a detecting device and a finger surface by placing the finger on the detecting device, and fingerprints are detected based on a capacitance difference due to unevenness on the finger surface constituting the fingerprints (hereinafter referred to as “fingerprint unevenness”) as a potential difference or a current difference. A second method is an optical method in which the detecting device irradiates light onto the finger surface, and fingerprints are detected based on a difference in light reflecting states due to the fingerprint unevenness. A third method is a pressure method in which the finger is pressed on the detecting device, and fingerprints are detected based on a difference in pressure due to the fingerprint unevenness, as a difference in electrical contact and non-contact or the capacitance difference. Among conventional methods including the above-described three methods, the capacitance method is simpler in terms of a structure in comparison to the other methods, and a section can be formed thinner, where the finger as a detection object is placed and the capacitance due to the fingerprint unevenness is detected. For this beneficial feature, the capacitance method is expected to be adopted in a portable terminal in the future.
As for the capacitance method, Publication No. EP0457398A2 (published on Nov. 21, 1991) and Japanese Unexamined Patent Application Tokukai No. 2000-213908 (published on Aug. 4, 2000), for example, disclose a device and a method for detecting the fingerprints adopting the capacitance method. A conventional uneven pattern detector is explained below, referring to a fingerprint detecting device disclosed in the Publication No. EP0457398A2, for example.
FIG. 10 is a partial sectional view of a fingerprint detecting device, which is the conventional uneven pattern detector. A basic structure of this fingerprint detecting device is prepared as follows. Formed on a substrate 101 is a circuit for detecting electric charges, which is composed of detecting electrodes 102 positioned in a matrix manner, address lines 103 and the like. Further, an insulation film 104 is formed so as to cover a top surface of the circuit. When a finger 105 is placed on the top surface of the detecting device, a distance d between a finger surface 106 and the insulation film 104 differs according to the fingerprint unevenness on the finger surface. Therefore, a capacitance 107 formed between the detecting electrode 102 and the finger depends on the distance d, and differs according to the fingerprint unevenness on the finger surface. The detecting electrode 102 receives a constant potential via the detecting circuit by way of switch elements (not shown) controlled by drive lines (not shown). At this point, the capacitance 107 charges or discharges in accordance with the potential of the detecting electrode 102. Then, by reading out charged or discharged electric charges from the detecting circuit, information is obtained indicating a pattern of the fingerprint unevenness.
In the foregoing conventional fingerprint detecting device, the fingerprint unevenness is reflected on changes in the charged electric charges of the capacitance 107, i.e. changes in the potential of the detecting electrode 102. Therefore, the potential of the detecting electrode 102 is desirably maintained at a value as constant as possible during a state before the electric charges are read out by switching ON the switch element, namely while the switch element is OFF.
On the other hand, Tokukai No. 2000-213908 discloses a capacitance detecting device, which is an uneven pattern detector that can be used for detecting the fingerprints, and which has a similar arrangement as the above-described fingerprint detecting device (the same reference symbols are assigned to the corresponding constituent members (see FIG. 10)). In the capacitance detecting device, a silicon substrate 101 is used, and the silicone substrate 101 is grounded. Therefore, a parasitic capacitance is formed between the substrate 101 and the detecting electrode 102. Because of this, the potential of the detecting electrode 102 is fixed by the capacitance 107 and the parasitic capacitance thereof, so that the potential of the detecting electrode 102 is stabilized.
However, the potential is stabilized because a substrate composed of a conductor (or a semiconductor) such as the silicone substrate is used for the supporting substrate 101. When an insulator such as glass is used for the supporting substrate, the supporting substrate 101 cannot be grounded. As a result, the potential of the detecting electrode 102 cannot be stabilized.
Further, when the parasitic capacitance as described above exists, charged electric charges according to the parasitic capacitance are added as an offset component to the charged electric charges according to the capacitance 107, which are to be detected. As long as the offset component is constant, it is possible to detect fingerprints by obtaining a difference in detected electric charges in accordance with the pattern of the fingerprint unevenness. However, this offset component narrows a range of the detected value of the charged electric charges, namely a dynamic range of the capacitance detecting device. Therefore, it is desirable that the offset component does not exist, or the offset component, if any, is not added to the detected electric charges.
In response, Tokukai No. 2000-213908 discloses a capacitance detecting device which prevents the charged electric charges according to the parasitic capacitance from being added as the offset component to the charged electric charges according to the capacitance 107, which are to be detected. In this capacitance detecting device, dummy electrodes are provided per each row of the detecting electrodes 102 for forming a cancel capacitance having a similar amount of the parasitic capacitance. However, when the dummy electrodes are provided, an arrangement of the detecting device becomes complicated. Further, the parasitic capacitance of each of the detecting electrodes 102 essentially differs respectively to some extent. Thus, when a constant area of the dummy electrode is provided with respect to the irregular parasitic capacitance so as to form the cancel capacitance, a certain amount of the offset component inevitably remains at each detecting electrode, thereby complicating a procedure for compensating the remaining portions.